1. Field of the Invention
The present invention relates to high-frequency modules for transmitting or receiving various types of high-frequency communication signals with a switch IC provided therein and an antenna and, more particularly, to a high-frequency module including an electrostatic discharge (ESD) protection device.
2. Description of the Related Art
Various high-frequency switch modules including a switch IC and a plurality of transmission circuits, reception circuits, and transmission/reception circuits which are connected to a single antenna via the switch IC have been proposed.
Such a high-frequency switch module typically includes a multilayer substrate. A switch IC is disposed on the multilayer substrate. An antenna transmission circuit, a transmission circuit, a reception circuit, and a transmission/reception circuit which are connected to the switch IC are defined by electrode patterns provided on the surfaces of the multilayer substrate, an electrode pattern provided in the multilayer substrate, and a surface-mounted component.
In high-frequency switch modules including a switch IC, in order to prevent the switch IC from being destroyed or damaged by a voltage surge externally applied thereto via an antenna, an electrostatic discharge protection device (hereinafter referred to as an ESD device) is disposed in a transmission circuit connecting the switch IC and the antenna. The ESD device is, for example, an inductor, and is connected between a transmission line arranged to connect the antenna and the switch IC and the ground. Since ESD devices typically must have a high withstand voltage, they are usually used as mounting components.
Since such an ESD device that is a mounting component and a switch IC are disposed on the surface of a multilayer substrate, it is necessary to provide a ground connection line arranged to connect the ESD device to the ground in the multilayer substrate.
As disclosed in Japanese Unexamined Patent Application Publication No. 2008-516494, a common ground electrode is disposed in a multilayer substrate and ground terminals of the switch IC and the ESD device are connected to the common ground electrode. The common ground electrode is connected to a common external-connection ground electrode disposed on the bottom surface of the multilayer substrate for external connection to the ground.
However, when both of the ESD device and the switch IC are connected to the common ground electrode, the following problem occurs. FIG. 8 is a schematic cross-sectional view describing a problem of a high-frequency switch module in the related art. In FIG. 8, only a pattern of lines connected to the ground is illustrated and the illustration of other line patterns is omitted.
A high-frequency switch module 10P in the related art includes a multilayer substrate 200P. An ESD device 110, a switch IC 121, and a SAW filter element 122 are disposed on the surface of the multilayer substrate 200P. Inner common ground electrodes 201P and 202P are provided in the multilayer substrate 200P. The inner common ground electrode 201P is connected to a ground land on which the ESD device 110 is disposed by a via hole 220, is connected to a ground land on which the switch IC 121 is disposed by a via hole 221, and is connected to a ground land on which the SAW filter element 122 is disposed by a via hole 222.
The inner common ground electrodes 201P and 202P are connected to each other by a plurality of via holes 230. The inner common ground electrode 202P is connected to an external-connection ground electrode 210 that is provided on the bottom surface of the multilayer substrate 200P by via holes 240.
As represented by a chain double-dashed line in FIG. 8, an external current surge flowing from the ESD device 110 to the external-connection ground electrode 210 enters the switch IC 121 and the SAW filter element 122 via the inner common ground electrode 201P. This causes a malfunction of the switch IC 121 and destroys or damages the switch IC 121 and the SAW filter element 122 which have low surge tolerances.